Big rewrite of Purifier class.
- move config from separate file to static variable in class (cascading config
files aren't too useful when 0 other modules are loaded).
- add ability to add new config group and HTMLPurifier instance if desired.
- add Purifier::process_input_array() to clean GPC superglobals.
Add superglobal XSS cleaning code (and purifier module) to bootstrap.